Mitochondria perform fundamental functions in eukaryotic cells, including ATP production via respiration and cellular ion and phospholipid homeostasis. They also serve as platforms to integrate signaling pathways such as cell death and innate immunity. Mitochondrial functions are tightly linked to mitochondrial form, established through separate, but somehow coordinated machines that control dynamics, positioning, motility and mitochondrial DNA (mtDNA) transmission. The endoplasmic reticulum (ER) has emerged as an integral and pervasive player in the regulation of mitochondrial form and function. The ER exerts its role through contacts with mitochondria, which we hypothesize create specialized microdomains, which can recruit and/or modulate effectors that control and integrate mitochondrial physiology with other organelles and signaling pathways. In most cases, however, the molecular composition of ER-mitochondria contacts is poorly defined and their exact mechanisms of action, their functional scope and modes of communication are poorly understood. In the aims of this grant, we will address these deficits by exploring the molecular basis and functions of different types of ER-mitochondria contact sites in yeast and we will extend our findings to mammalian cells using comparative and forward strategies. New information in this area of cell biology will provide insight into the general architecture and rols of ER contacts and their regulation of mitochondrial function and cellular homeostasis to more accurately reveal role of mitochondria in human diseases that result from mitochondrial and ER dysfunction.